Polyesters are used widely in various fields for fibers, films, resins and bottles because of their excellent properties. Among them, polyethylene terephthalate is favorably used since it is excellent in mechanical strength, chemical properties, dimensional stability, etc.
In general, polyethylene terephthalate is produced from terephthalic acid or any of its ester forming derivatives and ethylene glycol. In this case, in commercial processes for producing high molecular polymers, antimony compounds are widely used as polycondensation catalysts. However, polymers containing any antimony compound have the following several unpreferable properties.
For example, when a polyester obtained by using an antimony catalyst is melt-spun into fibers, it is known that the residue of the antimony catalyst is deposited around the holes of a spinneret. The reason why the residue of the antimony catalyst is deposited is considered to be that antimony exists as antimony glycolate in the polymer and that it is modified near the spinneret, and partially vaporized and dissipated, while a component mainly composed of antimony remains at the spinneret. If the deposit grows, it causes filament breaking, etc. and must be removed from time to time.
Furthermore, the antimony catalyst residue in the polymer is likely to be relatively large grains, and acts as a foreign matter, causing such unpreferable phenomena as filtration pressure rise at the time of processing, filament breaking at the time of spinning, and film breaking at the time of film formation.
Because of the above problems, a polyester having very small or zero in antimony content is being demanded.
To solve the problems, U.S. Pat. No. 5,512,340 and U.S. Pat. No. 5,596,069 propose to use an aluminum compound such as aluminum chloride or aluminum hydroxychloride and a cobalt compound together. However, in general, an aluminum compound has such problems that it is unlikely to be dissolved in a glycol such as ethylene glycol and the polyester reaction system, and if it is added to the polyester reaction system as a polycondensation catalyst, an insoluble foreign matter is formed to cause filament breaking at the time of spinning and film breaking. After all, the problems of antimony cannot be sufficiently avoided.